Open-loop amplifier control architectures, for example, in wireless communications transmitters, is desirable because they have reduced cost and complexity compared to closed-loop control architectures, which often include a coupler and power detector at the amplifier output. Open-loop amplifiers are generally less complex than closed-loop systems. In open-loop amplifier architectures, there is no feedback to the amplifier control signal during operation of the amplifier. Open-loop amplifiers also permit elimination of the commonly used coupler and detector, which add cost and consume power.
In some applications, for example, in wireless communications device transmitters, the load on the output of the amplifier can vary under certain operating conditions, and generally changes with the frequency of the amplifier signal, (i.e., the operating channel) for example, a carrier signal on which information is modulated. Particularly, the load on the amplifier output has characteristic impedance, which is generally frequency dependent. The changing load or impedance will also generally have some effect on the performance of the amplifier, for example, on the output power of a wireless communications transmitter. In many applications, the frequency of the amplified signal changes, for example, in frequency hopping modes of operation typical of some time division multiplexing (TDM) based communications protocols where signals having different frequencies are amplified during corresponding transmission bursts.
The various aspects, features and advantages of the disclosure will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description thereof with the accompanying drawings described below.